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Clearly
Superior
(Top) A 250-meter-long AUV sub-bottom profle across the shallow buried channel (vertical scale is referenced to the water surface using the AUV depth sensor).
imaging
SONAR
SYSTEMS
Horizontal layering (H) and dipping refectors (R) resolved in a 350-meter SBP line collected in the channel between Greenwich Bay and Patience Island, Rhode Island.
system, the small footprint from a lowfying AUV, combined with the strong
returns, results in positional accuracy
for small man-made features, such as
buried pipelines.
Confguring the AUV
The modular Gavia Offshore
Surveyor AUV consists of a nosecone, battery, control unit and propulsion unit. In commercial survey work,
the Offshore Surveyor AUV is normally
confgured with an inertial navigation
system (INS) assisted by a Doppler velocity log (DVL) and differential-ready
GPS receiver. Sensor modules are chosen dependent on the data desired,
with the vehicle automatically detecting their presence on start-up. The vessel can be confgured with one or two
lithium-ion battery modules that can
be exchanged in the feld in a few minutes once depleted, with the two-battery confguration allowing missions
up to 12 hours at 4-knot speed.
For the Greenwich Bay SBP missions, the build was: nosecone with
obstacle avoidance sonar and imaging camera, battery module, Kearfott
Corp. (Little Falls, New Jersey) T-24 INS
with Teledyne RD Instruments (Poway,
California) WHN1200 DVL, control
module with GPS, Teledyne Benthos
ATM-900 acoustic modem, Iridium
Communications Inc. (McLean, Virginia) A3LA-X telemetry, side scan sonar, SBP and propulsion unit. In this
confguration, the AUV was 2.9 meters
long and weighed 88 kilograms in air.
Mobilization
The Gavia Offshore Surveyor was
assembled and tested at the Teledyne
Benthos offces then transported by
road to Allens Harbor Marina near
North Kingstown, Rhode Island. Operations were conducted from the
University of Rhode Island RV Shanna
Rose, a 42-foot lobster boat adapted
for nearshore survey. It has a low back
deck suitable for manual AUV deployment in calm seas and an enclosed
survey offce for mission planning and
data processing. The AUV was manually carried onto the vessel in its cradle
and strapped down on the back deck
for transit to the survey site. Spare battery modules enabled a full day of operations without recharging.
Once at the survey site, the AUV
was manually deployed over the stern
of the vessel and manually piloted
away using the Wi-Fi link, and the mission execute command sent. During
the mission, the status of the vehicle
was monitored using the integrated
acoustic modem. Regular updates on
the vehicle position, depth and speed
were tracked on a laptop. Multiple
missions 45 minutes to two hours long
were run over the two survey days in
two separate areas of the bay.
SBP Data Collection
Initially, the AUV did a series of test
missions to determine the optimum
system settings and fy height for the
refector defnition and penetration in
Greenwich Bay. With an AUV-mounted SBP, the same survey line can be repeated accurately with data collection
settings changed in each run, enabling
tests of the effects of changing parameters, such as fy height, pulse length,
transmit power and receiver gain.
The frst set of missions consisted of
several lines across Greenwich Bay to
fnd a suitable sub-bottom feature for
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FEBRUARY 2013 / st
47